Conventionally, a construction machine such as a hydraulic excavator provided with a turbocharged engine generally develops a phenomenon called “lag-down”, that is, a temporary drop in engine speed when a hydraulic actuator is quickly operated and a sudden load is applied to the engine. This phenomenon takes place because, when the engine is not under load, the injection rate of fuel is low, the amount of exhaust gases that act on a turbocharger decreases to result in a low boost pressure, a response delay occurs in the rise of an engine torque, and therefore, a hydraulic load (a torque to be absorbed by a hydraulic pump) caused by the quick manipulation exceeds an engine torque. A temporary drop in engine torque due to such a lag-down manifests as a change in engine sound, so that an unpleasant feeling is given to an operator. Further, the delay in the rise of engine speed invites a decrease in the delivery rate of the hydraulic pump, and therefore, also affects work capacity and controllability. In addition, responsive to a reduction in engine speed, a governor increases the injection rate of fuel in an attempt to have the engine speed returned to a normal engine speed, thereby causing emission of black smoke and a deterioration of fuel economy by a quick injection of fuel.
Keeping in step with the move toward construction machines of higher performance in recent years, the maximum flow rate of a hydraulic pump and the maximum drive force of a hydraulic actuator tend to increase year by year, and therefore, the output required for an engine also has a tendency to increase. In an engine mounted on a construction machine in recent years, an increase in output is generally achieved by raising the turbo boost pressure. However, the engine torque during a decrease in supercharging has not changed much for years because it depends on the displacement of an engine. As a corollary to this, the engine torque during a decrease in supercharging has become increasingly different from an increase in engine output during supercharging, so that the time required for a rise in torque has become longer. As a consequence, it has become a problem in hydraulic control to prevent a lag-down that occurs by a quick manipulation as mentioned above.
For the above-described problem, it has been a conventional practice to choose an engine having a sufficient non-boost torque. However, an engine with a large non-boost torque has a larger size due to an increase in displacement or the like, thereby raising problems in cost, installation space and so on.
In the engine control system which is disclosed in JP-A-1-224419 and is useful for a civil engineering and construction machine, a reduction of lag-down is, therefore, achieved by arranging a means for applying a drag load during non-operation, increasing the engine torque to a certain extent to raise the turbocharged boost torque beforehand, and hence shortening the time required until an engine torque rises.